1. Filed of the Invention
This invention relates to an electronic sewing machine provided with an improved control system for controlling a DC motor employed for driving upper and lower shaft of the sewing machine. The control system will serve self-examination of a load applied to the sewing machine.
2. Description of the Prior Art
It has been recognized necessary that a loaded torque applied to an electronic sewing machine be within a predetermined reference range. The loaded torque may be tested on the manufacturing line of the sewing machine in such manner that a current value applied to the sewing machine is measured to determine wether the sewing machine is operating with a rated output. In case the applied current exceeds a predetermined limit, the sewing machine should be returned to a preceding step on the manufacturing line and re-assembled and re-tested.
FIG. 5 shows an example of prior art electronic circuits mounted within the sewing machine for self-diagonosis of the sewing machine load. In this circuit, a volume VR1 of a controller is connected to a power source Vcc whereby a voltage value which is controlled and changed depending upon an operating amount of the controller will be applied via a filter consisting of a resistor R1 and a capacitor C1 to an analog-digital converting terminal A/D1 of a one-tip control processing unit (hereinafter referred to as IC2). A pulse-width-modulation (PWM) signal which is corresponding to the voltage value thus determined will be outputted from IC2 for the control of DC motor M which drives the sewing machine. To a reset terminal of IC2 is connected an integrated circuit (IC1) for detecting every rise and fall of the power source Vcc to initialize IC2. The motor M is connected between another power source Vb and a drain of a power MOS-FET (FET1). In parallel with the motor M is connected a free wheel diode D1.
The resistor R5 is connected to a source of FET1 for detecting the current value input to the motor M. A signal representing a voltage which is in turn corresponding to the current value detected by the resistor R5 is supplied therefrom, via a filter consisting of a resistor R6 and a capacitor C3, to an analog-digital converting terminal A/D3 of IC2. There is provided a diode D2 for preventing an excessive voltage from being inputted to the terminal A/D3. The PWM signal outputted from IC2 is supplied via a gate IC3 and a gate protecting resistor R4 to a gate of FET1.
A sensor 10 is provided for detecting a rotating speed of the motor and supplying a detection signal toward a port P1 of IC2. The sensor 10 may comprise, as known, a disc plate provided with a slit and secured to the sewing machine upper shaft or a main drive shaft and a photo-interrupter device cooperating with the slitted disc to detect a rotating speed of the shaft, thereby detecting the rotating speed of the motor M.
There are also provided a potential divider consisting of two resistors R2 and R3 for detecting a voltage supplied from the motor drive power source Vb. A divided voltage obtained at a connecting point between these resistors R2 and R3 should be set to a value which does not exceed 5 V, a voltage value supplied from the control power source Vcc, and which is supplied to the terminal A/D2 of IC2. A capacitor C2 serves as a filter for the divided voltage value.
With the prior art system shown in FIG. 5, the current value input to the sewing machine motor M can be detected by the resistor R5. However, when a current flow is supplied to the resistor R5 which is connected in series with the motor M, a source potential is increased so that a gate voltage is relatively decreased, thereby making it impossible to drive FET1. To cope with this problem, it has been necessary to provide another power source Va having a voltage value, e.g. 12 V which is substantially higher than 5 V of the control power source Vcc, to be supplied to the gate of FET1. An additional problem that the prior art has incurred is that the current detecting resistor R5 should be of a considerably great degree of capacity. Supposing the resistor R5 will detect a voltage value of about 1V when the sewing machine is driving with a rated current of the DC motor which is determined to be about 1A, the resistor R5 will have a resistance of 1.OMEGA.. However, under the locked condition of the sewing machine, the resistor R5 would receive an overcurrent of at least 10A, resulting in heat generatation of (10A).sup.2 .times.(1.OMEGA.)=100 W or more. In an actual system, there is provided a safe guard means for cutting off the electricity supply to the motor in a while responsive to detection of the locked condition of the sewing machine so that a capacity of the resistor R5 may be decreased to the order of a fraction of 100 W. Even so, the current detecting resistor R5 has still been required to have a considerable capacity.